Catalytic converter manufacturing method

ABSTRACT

A method for manufacturing a catalytic converter having a ceramic substrate comprises sizing a housing of a catalytic converter over a substrate and intumescent mat subsequently to heating the converter, the heating causing the intumescent mat to at least reach a temperature at which the intumescent mat swells.

BACKGROUND

[0001] The present disclosure relates to catalytic converters fortreatment of exhaust, e.g., from internal combustion engines. Moreparticularly, the disclosure relates to a method of manufacturingcatalytic converters having fragile substrates.

[0002] Newer “thin-wall” substrates in catalytic converters offersignificant advantages over traditional catalytic converter substrates,including a greater geometric surface area per unit volume and fasterlight-off due to the lower thermal mass of the substrate. As isgenerally understood in the art, faster light-off translates to higherconversion efficiency since catalytic converters are not effectiveduring a cold engine start until they reach operation, or light-offtemperature. However, thin wall substrates are significantly morefragile and are subject to fracture during stressful manufacturingoperations that including stuffing, sizing, and burnoff operations.

[0003] One method currently employed to manufacture a catalyticconverter includes wrapping the substrate in a matting material, or mat,and stuff the substrate and mat into a metal housing through the use ofa stuffing cone, the cone serving to compress the matting so that it canslide into the housing (see FIG. 1). The mat serves to support thesubstrate, insulate the housing from the high temperatures reachedwithin substrate, and protect the substrate from shocks and vibrations.Although converter housings are most often cylindrical, they can beother shapes as well, such as having elliptical or oval cross sections.

[0004] Depending on the type of mat, whether it is intumescent ornon-intumescent, the necessary stuffing pressure varies. Intumescentmats are called such because they swell under high temperature. Thisswelling is a property of a component of the mat, typically vermiculite.Non-intumescent mats do not contain vermiculite. These matting materialsare well known in the art and are available from 3M, Minneapolis Minn.as well as from Unifrax Co., Niagra Falls, N.Y. The swelling property ofintumescent mats is useful because it helps to maintain a positivepressure between the substrate and the housing during the thermal cycleimposed on the converter in normal use. In use, the diameter of themetal housing increases due to thermal expansion to a greater degreethan that of the ceramic substrate. Thus, to maintain a positivepressure, it is advantageous to employ a mat that swells to fill thegrowing gap as the temperature rises. Non-intumescent mats must bestuffed under much greater force to a high level of compression in orderto ensure a continued positive pressure between the substrate andhousing during use. This high-force stuffing is more time consuming andtakes considerable energy, which significantly increases the overallproduction cost of the converter.

[0005] Once the substrate and matting material is stuffed into thehousing, the housing may be sized and appropriate connections are formedfor assembly into an exhaust system. Sizing operations, when necessary,compensate for variations in substrate diameters, and may comprisecompressing the housing to produce an overlapped seam, and then welding,or a housing may be reduced by drawing or compressing the housing usinga pipe-sizer.

[0006] Exhaust pipe connections may be formed in or welded onto eitherend of the housing. The connections include portions having varyingcross-sections to conform the stream entering the converter to the shapeof the substrate, thereby allowing exhaust to flow smoothly from theengine into and out of the converter, and through the remaining exhaustsystem to the tail pipe.

[0007] After the exhaust pipe connections are formed on the housing, theconverter is ready to be assembled into an engine. During theconverter's first use, the converter is heated to normal operatingtemperature, which may be anywhere from 300° C. to more than 500° C.This first use or heating drives off organic binders within the mat andcauses the intumescent material within the mat to greatly expand, thusincreasing the pressure within the confines between the housing andsubstrate. Some substrates, particularly the newer, more fragilesubstrates, can fail under this pressure, rendering the entire converterunusable.

[0008] A graph showing estimated matting pressure verses time during themanufacturing method described above is provided in FIG. 5. Starting atthe left side of the graph, the intumescent matting is stuffed undervery low pressure, i.e., less than 10 pounds per square inch (psi). Thepressure is greatly increased to about 150 psi during the sizingoperation. After sizing, the matting responds by relaxing somewhat,reducing the stress therein and the pressure to about 100 psi. Theexhaust pipe connections are then formed in or welded to the housing,which does not affect the pressure of the matting. Finally, theconverter is heated, e.g., during its first use, which causes swellingof the matting, which increases the pressure by about 80 psi to 180 psi.The pressure may be even higher locally within the matting material dueto variations in the matting or the substrate itself. The fragileceramic substrate is sometimes unable to stand up to these highpressures and fail.

[0009] It would be desirable to reduce the likelihood of breaking theceramic substrate during the production or first use of a catalyticconverter.

SUMMARY

[0010] The above-discussed and other drawbacks and deficiencies of theprior art are overcome or alleviated by sizing a housing of a catalyticconverter over a substrate and intumescent mat subsequently to heatingthe converter, the heating causing the intumescent mat to at least reacha temperature at which the intumescent mat swells.

[0011] The above-discussed and other features and advantages of thepresent invention will be appreciated and understood by those skilled inthe art from the following detailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The present invention will now be described by way of examplewith references to the accompanying drawings, in which:

[0013]FIG. 1 shows a flow chart illustrating a method to produce acatalytic converter;

[0014]FIG. 2 shows a first step in the construction of a catalyticconverter, according to one embodiment of the disclosed method;

[0015]FIG. 3 shows a subsequent step in the construction of a catalyticconverter, according to one embodiment of the disclosed method;

[0016]FIG. 4 shows a subsequent step in the construction of a catalyticconverter, according to one embodiment of the disclosed method;

[0017]FIG. 5 shows a pressure-time diagram illustrating the advantagesof the method shown in FIG. 1; and

[0018]FIG. 6 shows a pressure-time diagram illustrating the disadvantageof another method.

DETAILED DESCRIPTION

[0019] A flow chart diagramming a method for manufacturing a catalyticconverter is shown in FIG. 1. The method will be described to someextent by reference to FIGS. 2, 3, and 4. It has been found that byheating the material prior to sizing reduces the internal mattingpressure against the fragile substrate and therefore reduces thelikelihood of breakage thereof.

[0020] Thus, in the preferred process, in a first step 12, intumescentmatting 27 is wrapped around a substrate 25, and the substrate 25 withmatting 27 is stuffed into a housing 29. This operation is ordinarilyconducted through the use of a stuffing cone 30, as shown in FIG. 1. Thestuffing cone compresses matting 27 to a diameter the same as orslightly smaller than the smallest potential diameter of housing 29,according to manufacturing tolerances, thus allowing matting 27 andsubstrate 25 to slide into place within housing 29.

[0021] The stuffing operation is done under low pressure, and low mountGap Bulk Density (GBD). The GBD defines the level of mat compression ingrams per cubic centimeter. The preferred mount density for the stuffingoperation is less than about 0.7 g/cm³. It is also preferred that themount density be greater than about 0.6 g/cm³.

[0022] After stuffing, substrate 25 and matting 27 are positioned withinhousing 29 as shown in FIG. 2, and the assembly is heated in an oven toundergo burn-off and expand step 14 (FIG. 1). It is preferred that theassembly be heated to a temperature greater than about 500 degreesCelsius, During this step, organic binders are burned off and thevermiculite or other intumescent component of matting 27 swells.

[0023] Following burn-off/expand step 14, housing 29 is sized by step 16to bring the GBD to approximately 1.0 g/cm³. The target size for eachhousing may vary depending upon the size of the substrate.Alternatively, the target size may be a dimension that is common for allconverters being manufactured that is optimally determined to satisfyGBD requirements within reasonable tolerances.

[0024] There are several known methods for sizing housing 29. Apreferred sizing technique is described in commonly-assigned U.S. patentapplication Ser. No. 09/141,299, filed Aug. 27, 1998 by Michael R.Foster, et al., which is incorporated herein by reference in itsentirety. In this method, each substrate 25 is individually measured todetermine its dimensions prior to stuffing into a housing that does nothave any slits. The housing is then sized by compressing it from alldirections in a radial press, thereby plastically deforming the housinguntil it reaches the target size. Such sizing devices are generallyknown for expanding and diametrically compressing pipes.

[0025] Another method that can be used, sometimes referred to as atourniquet or shoebox method, is best for cylindrical housings havingone or two slits and includes compressing or tightening the housing,e.g., with a strap or press, until the proper size is reached or thecompressing force reaches a selected stop force, then welding the seemor seams. Housing 29 is reduced in size until the target size is reachedor a selected stop-force sensed by the sizing machine. In this manner,sizing step may compensate for variations in the size of the substrate.

[0026] After the sizing step 16, mat 27 undergoes a relaxation step 18in which the mat material partially relaxes, reducing the internalpressure. Finally, the connection ends 32 are added to housing 29 byform/weld step 20. Form/weld step 20 may comprise any known method offorming connection ends onto housing 29, either by welding them to thehousing 29 or by deforming housing 29 to shape the connection ends. Inone preferred embodiment, housing 29 extends some distance on eitherside of substrate 25 as shown in FIG. 2, and undergoes a spin-formprocess in which rollers progressively shape either end until connectionends 32 are formed as seen in FIG. 3.

EXAMPLE

[0027]FIG. 4 shows a diagram showing estimated pressure changes in anexample according to the method described above. Initial mountingpressure is shown at the left side of the diagram to be less than 10psi. Burn-off/expansion increases the pressure to about 80 psi. Notethat a similar pressure increase occurs in the current process shown inFIG. 5 during the burn-off/expand step 14. Subsequent to the burn-offexpand step 14, pressure increases again during sizing step 16, duringwhich internal mat pressure increases to about 150 psi. This samepressure is attained in the current method shown in FIG. 5 because, asnoted above, the internal pressure subsequent to the sizing operation isdependent upon the GBD of the mat. Relaxation step 18 reduces internalpressure by about 50 psi to about 100 psi. Since matting 27 has alreadybeen expanded in expansion step 14, subsequent heating in use of thedevice will not increase the pressure within the mat to such a degreethat substrate 25 is likely to fail.

[0028] While preferred embodiments have been shown and described,various modifications and substitutions maybe made thereto withoutdeparting from the spirit and scope of the invention. Accordingly, it isto be understood that the present invention has been described by way ofillustration and not limitation.

I claim:
 1. A method of manufacturing catalytic converters comprising:sizing a housing of a catalytic converter over a substrate andintumescent mat subsequently to heating said converter, said heatingcausing said intumescent mat to at least reach a temperature at whichsaid intumescent mat swells.
 2. The method of claim 1, said methodfurther comprising: wrapping said substrate with said mat, forming awrapped substrate and stuffing said wrapped substrate into said housingprior to said heating.
 3. The method of claim 2 wherein said stuffingcomprises increasing the gap bulk density to a mount density no morethan about 0.7 g/cm³.
 4. The method of claim 1 wherein said sizingcomprises compressing said housing over said substrate to a GBD of about1.0 g/cm³.
 5. The method of claim 1 further comprising allowing said matto relax subsequent to said sizing.
 6. The method of claim 1 furthercomprising, adding connection ends on said housing subsequent to saidsizing, said adding comprising one of forming and welding saidconnection ends.
 7. The method of claim 1 further comprising allowingsaid mat to relax subsequent to said sizing, and adding connection endson said housing subsequent to said sizing.
 8. The method of claim 7wherein said allowing and said adding occurring concurrently.
 9. Themethod of claim 7 wherein said adding comprises plastically deformingsaid housing to form one of said connection ends.
 10. The method ofclaim 7 wherein said adding comprises welding one of said connectionends onto said housing.
 11. The method of claim 1 further comprising:wrapping said substrate with said mat, forming a wrapped substrate;stuffing said wrapped substrate into said housing prior to said heating;allowing said mat to relax subsequent to said sizing; and addingconnection ends on said housing subsequent to said sizing by one offorming and welding said connection ends.
 12. The method of claim 11wherein said stuffing comprises increasing the gap bulk density to amount density no more than about 0.7 g/cm³.
 13. The method of claim 12wherein said heating comprises heating said converter to at least about500° C.
 14. The method of claim 1 wherein said heating comprises heatingsaid converter to at least about 500° C.
 15. The method of claim 14wherein said heating comprises heating said converter to a maximumtemperature less than about 600° C.